The white light LED (Light-emitting diode, LED) technology can be divided into single-chip type and multiple-chips type. The multiple-chips type using LEDs of three colors to mix the light emitted respectively into the white light. The benefit of the multiple-chips type is the flexibility of the light mixing based on different visual demand. However, the cost is relatively high for using many of LEDs chips at the same time. Furthermore, because the materials of making the LEDs with various colors are different, which needs different forward voltage (Vf) of the input, three sets of control circuit are needed. In addition, the speed of decay, the temperature characteristics and the life-time of the three kinds of LED chips are not the same, so the color of the white light is not stable and changes with time.
For single-chip type, there are three ways to manufacture the commercialized white light LED: 1. Blue LED chips combine with the yellow phosphor: The yellow phosphor used mainly is YAG phosphor of the yttrium aluminum garnet structure. The yellow light converted by the phosphor mixed with the unabsorbed blue light emitted from the blue LED can generate the white light. 2. Blue LED chips combine with the red and green phosphors: The phosphor used here is mainly containing sulfur. The red and green lights from the phosphor are combined with the unabsorbed blue light, and a white light is generated. 3. Ultraviolet LED chips combine with the red, blue and green phosphors: The ultraviolet light generated by the UV LED chips excites three or more kinds of phosphors which can respectively generates the light of red, blue and green at the same time, and the mixture of the lights mentioned above can generate the white light. To manufacture the above-mentioned devices, the phosphor and silicone is disposed into a concave after being mixed. However, it is difficult to mix the phosphor and silicone uniformly. As a result, the color temperature is uneven. When the light enters the phosphor-silicone layer from the LED, a high proportion (for example greater than 30%) of light can be back-scattered into the chips by the phosphor. Therefore, the light utilization efficiency is reduced.
U.S. Pat. No. 6,642,652 disclosed a flip-chip semiconductor light-emitting device covered by the phosphor structure formed by electrophoresis. But the method is difficult to distribute the powder uniformly in chip scale by controlling the electric field distribution and the electric charged state of the inorganic powder body, especially in the chip sidewall.